A secondary lithium ion battery is a rechargeable power source that can be implemented into a wide variety of stationary and portable applications. The structure and electrochemical reaction mechanism of this type of battery provide it with several desirable characteristics including a relatively high energy density, a relatively low internal resistance, a general non-appearance of any memory effect when compared to other types of rechargeable batteries, for example, a nickel-cadmium battery, and a low self-discharge rate. These characteristics have made the lithium ion battery the preferred mobile power source for portable consumer electronics such as laptop computers and cell phones. Larger-scale versions that interact with a multitude of interconnected systems have also been designed and manufactured by the automotive industry in an effort to improve vehicle fuel efficiency and reduce atmospheric pollution. The powertrains of hybrid electric vehicles (HEV) and extended range electric vehicles (EREV), for example, rely on the cooperative effort of multiple lithium ion batteries and a hydrocarbon-fueled internal combustion engine to generate power for vehicle operation.
A lithium ion battery generally contains one or more electrochemical battery cells that include a negative electrode, a positive electrode, and a porous separator sandwiched between confronting inner face surfaces of the electrodes. Each of these battery components is wetted with a liquid electrolyte solution that can communicate lithium ions. The negative and positive electrodes are formed of different materials that can intercalate and de-intercalate lithium ions and, when connected, establish an electrochemical potential difference. An interruptible external circuit connects the negative electrode and the positive electrode to provide an electrical current path around the separator to electrochemically balance the migration of lithium ions through the separator between the electrodes. Metallic current collectors intimately associated with each electrode supply and distribute electrons to and from the external circuit depending on the operating state of the electrochemical battery cell. The external circuit can be coupled to an electrical load (during discharge) or an applied voltage from an external power source (during charging) through conventional electronic connectors and related circuitry.
The porous separator includes opposed major face surfaces that intimately contact the confronting inner face surfaces of the electrodes. A main function of the separator is to provide a porous and electrically insulative mechanical support barrier between the negative and positive electrodes to prevent a short-circuit. Conventionally, the porous separator has been composed of a polyolefin such as polyethylene and/or polypropylene. A number of fabrication methods have been developed for making a polyolefin separator with its intended porosity. The separator may be formed by a dry technique in which a polyolefin polymer is melted, extruded into a film, annealed, and then uniaxially stretched. The separator may also be formed by a wet technique in which a polyolefin polymer is mixed with a hydrocarbon or other low-molecular weight liquid substance. The mixture is then heated, melted, extruded into a sheet, and biaxially stretched. Afterwards, the hydrocarbon or other low-molecular weight liquid substance is extracted.
A polyolefin separator, however, is potentially susceptible to certain performance declines when heated excessively. Exposure of the electrochemical battery cell to temperatures of 80° C. and above can cause the polyolefin separator to shrink, soften, and even melt. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source. The physical distortion of the polyolefin separator may ultimately permit direct electrical contact between the negative and positive electrodes and cause the electrochemical cell to short-circuit. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent. This inability of a polyolefin separator to maintain thermal stability at temperatures exceeding 80° C. for prolonged periods is a potential concern for some lithium ion battery applications.
Several engineering polymers that exhibit better thermal stability than polypropylene and polyethylene have been investigated as candidates for the separator in an effort to enhance the temperature operating window of a lithium ion battery. But the separator fabrication methods often used for polyolefin separators generally cannot provide these types of polymers with a sufficient and uniform porosity across their thickness at reasonable costs. The stretching techniques often employed in conventional polyolefin separator manufacturing processes has also been shown to adversely affect the dimensional stability of engineering polymer separators at elevated temperatures above 80° C. and, more noticeably, above 100° C. A fabrication method that can reliably produce, from a variety of engineering polymers, a thermally stable polymer separator having a generally uniform network of pores defined across its thickness is therefore needed.